This invention generally relates to a system for protecting a gas purification system from damage. In particular, this invention is directed to a process and system for operating an ultra high purity gas purifier using a getter while minimizing the chance for damage to the gas purification system.
Ultra-high purity (UHP) gas purification systems are used to supply customers with UHP nitrogen. The initial source of nitrogen (the distillation plant, or the liquid nitrogen supply) typically contains impurities including about 1 ppm oxygen by volume. The oxygen level is checked by an oxygen analyzer before passing into the gas purification vessel, which contains material that reacts with impurities in the nitrogen to produce purified gas.
Ultra-high purity inert gas purification systems generally employ a chemically-reactive getter metal that is comminuted by some means, and then dispersed in a matrix of a comparatively inert substrate material (usually alumina or similar). Once activated by a reduction process of some kind, this high surface area metal can then react extremely rapidly with various impurity gases (oxygen, hydrogen and others, depending on the getter material and temperature) to chemically bond with such gases, and so remove them from the gas stream: a process known as chemisorption.
The speed of the reaction, plus its highly exothermic (heat-evolving) nature means that processing inert gases containing high levels of a reactive impurity may cause significant damage and personal danger. For example, it is known that exposing activated nickel-based getters to oxygen concentrations greater than 1% will generally cause heat-damage to the catalyst, and possibly also to the reactor vessel and downstream customer equipment. The cost of the damage in such an instance ranges from tens of thousands to millions of dollars, when accounting for the impact on downstream processing.
One safety scheme is to measure the temperature of the catalytic bed using judiciously placed temperature measuring devices, such as thermocouples. If the bed temperature rises due to exothermic reactions, action is taken to safeguard the bed. This action will typically consist of diverting the feed gas and venting the purifier to rid it of remaining reactive gases and preventing additional reactive gases from entering. This will typically be performed automatically using a control unit that recognizes that a temperature setpoint has been reached and actuates valves to the shutdown condition. A major problem with this approach is that it requires that the bed be exposed to high levels of reactive impurities before action is taken, since this is necessary to raise the temperature of the bed.
Another safety scheme is to sample the gas stream prior to entering the bed. When a predetermined level of contaminants is reached, typically orders of magnitude above that normally present in the feed gas, action is taken to safeguard the bed. There are several types of impurity-level monitoring device. Typically, commercially available gas analyzers can be used. The approach of measuring the gas stream prior to entering the bed has the advantage that it can potentially allow more rapid reaction than an approach using thermocouples placed inside the purifier that is being protected, i.e., it can take action to safeguard the bed before the bed is exposed to high levels of reactive species. Further, it is possible to detect levels of reactive species that are higher than normal operation but are below that required to raise the temperature of the bed significantly. Thus, it is possible to design a system that is more sensitive to reactive species than one that simply embeds thermocouples inside the purifier bed and waits for these to register an increase in temperature.
One drawback to this approach is the cost of the unit, both in terms of the initial purchase cost and the maintenance required to keep the analyzer in good working order. For example, oxygen is commonly the species from which the purifier must be protected. Two standard varieties of oxygen-detection cell provide an electrical current output from either (i) a cell that operates at ambient temperature, containing salt solution that needs to be maintained at a fairly constant level by continuous replenishment with deionized water or new salt solution, or (ii) a zirconia (ZrO2) cell maintained at high temperature (greater than 600xc2x0 C.), that has a typical lifetime of two years or less.
The cost of using analysis external to the bed is compounded by the need to protect the gas from a backflow condition. Backflow may occur due to errors in operation or piping hook-up, or as a result of upset conditions that cause the pressure within the purifier to be lower than the pressure in what is usually the downstream direction and thus cause gas to enter the bed in a direction counter to that during normal operation. Protection against backflow requires that both the stream entering and leaving the purifier must be sampled. This increases cost and reduces system reliability by requiring two measuring devices as opposed to one.
It is therefore a priority to ensure (by monitoring) that such instances are avoided. An ideal system to achieve (or monitor) this will have three main features. It must 1) respond rapidly to increases in impurity level; 2) be easily maintained and operated, and 3) be cost effective enough to allow several redundant analyzers to be employed to monitor reactive impurities in the gas stream. This redundancy guarantees superior reliability, which is necessary to minimize the risks, both personal and financial, as discussed above.
Some prior art references have attempted to provide a system that attempts to safeguard the getter. U.S. Pat. No. 6,168,645B1 discloses a safety device located both up and downstream of the getter. The safety device contains getter material, along with thermocouples. This patent discloses a method of detecting impurities in gases by their reaction with a purification material that exhibits an exothermic reaction when an impure gas is passed over it. The resultant change in temperature is then detected by various means, including measuring the temperature of the gas using a thermocouple, or melting of the purification material. Once a certain temperature has been reached, a control system is then triggered, to cause it to carry out remedial action.
However, the ""645 patent has a significant disadvantage in that a continual low level of impurity will eventually consume the purification material, and will cause a gradual reduction in both the speed and the level of response (smaller change of temperature) to increases in the impurity concentration. For example, it is well known in the industry that a container filled with activated nickel getter has a limited capacity to react with oxygen, hydrogen and other impurities. The oxidation reaction is:
Ni+xc2xd O2-- greater than NiO+heat 
However, once all the nickel has reacted in this way, there will be no further heat output by the purification material, regardless of how high the oxygen concentration is in the gas or other fluid passing over it.
It is believed, therefore, that the ""645 patent may be practiced only if there is the capability for either regeneration or replacement of the purification material. The condition of the purification material must itself be monitored to ensure that the safety device will give the appropriate response when exposed to excessive amounts of impurity.
U.S. Pat. Nos. 6,068,685 and 6,156,105 disclose protecting the purifier both upstream and downstream. A first temperature sensor is disposed in a top portion of the getter material that constitutes the purifier bed. The first temperature sensor is located in a melt zone to detect rapidly the onset of an exothermic reaction which indicates the presence of excess impurities in the incoming gas to be purified. A second temperature sensor is disposed in a bottom portion of the getter material. The second temperature sensor is located in a melt zone to detect rapidly the onset of an exothermic reaction, which indicates that excess impurities are being backfed into the getter column. Action is taken to protect the bed if the setpoint is exceeded in either thermocouple.
However, the ""698 patent does not disclose any means of preventing impurity intrusion into the getter reaction vessel. Catalyst is not employed. A reactant/impurity reaction is not employed. The differential temperature of input gas and output gas is not measured, only the absolute temperature of the getter material at the top and/or bottom of the vessel.
A technique known in the art to detect hydrogen at above its lower explosive limit (LEL) in air is from a MSA Instrument Model 5100 instrument. This instrument uses a pair of heated filaments, one of which is surface-treated with catalyst to speed the oxidation of fuel-gas present in air. This instrument, however, does not disclose the addition of a reactant to a fluid stream, but merely relies on the natural presence of a stoichiometrically-excessive amount of oxygen in air to act as the oxidizing reactant.
There is therefore a need in the art to prevent damage to the getters during the purification process. As such, safety schemes have been devised to protect the bed from excessive levels of contaminant, and thus excessively high temperatures.
A process for purifying an impure gas to produce a purified gas in a gas purification system and protecting the system from damage comprising passing a first impure gas stream into a data analyzer flow scheme and a gas purification apparatus; passing a second reactant-containing gas stream into the analyzer flow scheme; mixing the first impure gas stream with the second reactant-containing gas stream to form a mixed gas stream; passing the mixed gas stream to a first temperature measuring device to determine its temperature and passing the resulting temperature data to a data analyzer; passing the mixed gas stream from step d) to a catalytic bed to allow the reaction in the mixed gas stream to proceed and forming a reacted mixed gas stream; passing the reacted mixed stream to a second temperature measuring device to determine its temperature and passing the resulting temperature data to the data analyzer; and controlling the flow of the first impurity containing gas stream passing to or from the gas purification apparatus based on data received from the data analyzer. In one particular embodiment, a plurality of measuring devices and catalytic beds in parallel is used to determine the temperature of the reaction in the catalytic beds. Preferably, two or more sets of measuring devices and catalytic beds are used in parallel.
In another embodiment, this invention is directed to a process for purifying an impure gas to produce a purified gas in a gas purification system and protecting the system from damage, comprising passing a first impure gas stream into a data analyzer flow scheme and a gas purification apparatus; passing a second reactant-containing gas stream into the analyzer flow scheme; mixing the first impure gas stream with the second reactant-containing gas stream to form a mixed gas stream; separating the mixed gas stream into a plurality of split streams; passing one of the split stream to a first temperature measuring device to determine its temperature and passing the resulting temperature to a data analyzer; passing the resulting split streams from the previous step to a reaction vessel to purify the resulting split stream; passing the resulting split stream from the previous step to a second temperature measuring device to determine its temperature and passing the resulting temperature to the data analyzer; repeating the previous three steps with another split stream through corresponding temperature measuring devices and reaction vessels; and controlling the flow of the first impure gas stream passing to the gas purification apparatus based on data received from the data analyzer.
The present invention is also directed to a system for purifying an impure gas and protecting a gas purification system from damage comprising a first impure gas stream; a second reactant-containing gas stream; a reactor vessel; a plurality of temperature measuring devices to measure the temperature of the mixture of first impure gas stream and second reactant-containing gas streams before and after the gas flow in the reactor vessel; and a data analyzer for analyzing the temperature difference of the mixture of first impure gas stream and second reactant containing gas streams before and after the gas flow in the reactor vessel and controlling the flow of the first impure gas stream.
Both the first impure gas stream and the second reactant containing gas stream pass through flow control devices and pressure gauges. The temperature measuring device is preferably a thermocouple, but may be any device that can accurately measure the temperature of a gas. The reaction vessel may comprise a catalyst bed.
As used herein, a getter is defined as a solid material (usually metal) that reacts irreversibly with impurities (xe2x80x9cgetteringxe2x80x9d) in a fluid stream to eliminate those impurities from the stream.
As used herein, activation is defined as a process whereby the reaction products from the gettering process are removed from the getter, to allow the getter to be used once more to clean up the gas.
As used herein, a catalyst is a material that speeds up the establishment of an equilibrium.
As used herein, ultra-high purity (or UHP) is a term used to describe a gas that contains less than about 10 ppbv (parts per billion by volume) of each impurity.